How many instances of Kontakt are you running? if one only, you are overloading one of the four cores. You need to balance your Kontakt instruments among the four cores by running four instances and assigning your instruments accordingly. 

As you've learned, Logic's MusicXML import leaves a lot to be desired. Logic has its strengths but it's primary purpose is to sell Apple computers.

Are you running SSDs only or still using an HHD?

i would max out the RAM in your iMac. When I went to 32G, mine ran better.

I'm now using separate instances of Kontakt. One for brass, one for woodwinds, one for mallet percussion,

and two for all the rest. That gives me a total of 5 Kontakt instances, and a extra instance running Engine

for Saxes. After some  trial and error, I lowered the max voices for all of the horn stuff and purged all of the

sample tracks to get rid of unused samples from loading and draining my system. All of my sample libraries 

'are on multiple external usb HHD's. I haven't gotten around to acquiring any SSD drives. Next on the 

never ending studio equipment list. Everything is running just fine now, with no dropouts or hanging notes.

 I wish that XLM import/exporting was more fool proof, but maybe in time. I also tried to use Finale for notation and Logic

as the audio source by sending midi out of Finale to control Logic Pro's audio. Still way too many bugs and glitches to make

it a viable solution at this time.

 I have something that works for now so we're all good. 

What year is your iMac? 21.5" or 27" i7 or i5" internal flash or HHD?

My iMac is the late 2013 27" Quad Core i7, 3.5Ghz processor with 3TB internal HHD with 24gb of Ram and a Focusrite Scarlett 18i0 audio interface.

I also have several 3TB external Hard drives just for sample libraries and a dedicated Hard drive for Time Machine. I plan on getting individual hard drives specifically for  my larger capacity sample libraries. One each for Komplete Ultimate, Composer Cloud, and VSL. The rest will remain on

on my current external drives. At least that is the plan. I'm just getting into the VSL stuff.

Good machine but can made be a lot better for not much$. If you are intent on using Kontakt, you should consider changing your internal to a 2T SSD. The Crucial MX300, the correct OWC heat sensor, bracket and foam tape will run about $600. This takes about an hour for an experienced tech, longer at home because the foam tape residue needs to be cleaned up. Since yours is 4 years old, replacing the BR2032 NV RAM battery at the same time is not a bad idea (adds about 2 minutes to this job). Besides the fact that everything will run much, much (much) faster, the energy savings between replacing the HHD and not running the fans to cool it will more than pay for itself—it certainly did when I performed the same operation in my 2010.

Your library drives should also be SSDs—USB 3 is ok but Thunderbolt is better.

Maxing your RAM to 32G is still a very good idea.